IrO2/MnO2 metal oxide-support interaction enables robust acidic water oxidation

The sluggish kinetics, poor stability, and high iridium loading in acidic oxygen evolution reaction (OER) present significant challenges for proton exchange membrane water electrolyzers (PEMWE). While supported catalysts can enhance the utilization and activity of Ir atoms, they often fail to mitigate the detrimental effects of over-oxidation and dissolution of Ir. Here, we leverage the redox properties of the Mn3+/Mn4+ couple as electronic modulators to develop a low-iridium, durable electrocatalyst for acidic OER. Specifically, IrO2 nanoparticles are anchored onto MnO2 nanowires (denoted as IrO2/MnO2), through a molten salt-assisted synthesis method. This optimized IrO2/MnO2 electrocatalyst features a substantially reduced iridium content and enhanced electronic structure due to strong metal-support interactions. Remarkably, the IrO2/MnO2 catalyst demonstrates 7-fold increase in intrinsic activity and superior durability compared to commercial IrO2. Both theoretical and experimental results indicate that dynamic electron transfer between Ir and Mn facilitates the rapid formation of highly oxidized iridium sites while simultaneously preventing excessive oxidation, thereby enhancing both the kinetics and stability for OER. A PEMWE utilizing IrO2/MnO2 as the anode catalyst achieves 2000 mA cm-2 @ 1.89 V without requiring supporting acidic electrolyte. Importantly, the PEMWE exhibits negligible degradation under harsh industrial operating conditions (1000 mA cm-2) with an Ir loading as low as 0.5 mg cm-2, while maintaining a low energy consumption of 45.58 kWh kg-1 H2, corresponding to the green hydrogen production cost of $0.9 kg-1 H2, significantly lower than the 2026 US-DOE target, underscoring its potential for practical application.

Quaternary Mixed Oxides of Non-Noble Metals with Enhanced Stability during the Oxygen Evolution Reaction

Robust electrocatalysts required to drive the oxygen evolution reaction (OER) during water electrolysis are still a missing component toward the path for sustainable hydrogen production. Here a new family of OER active quaternary mixed-oxides based on X-Sn-Mo-Sb (X = Mn, Fe, Co, or Ni) is reported. These nonstoichiometric mixed oxides form a rutile-type crystal structure with a random atomic motif and diverse oxidation states, leading to the formation of cation vacancies and local disorder. The successful incorporation of all cations into a rutile structure was achieved using oxidizing agents that facilitates the formation of Sb5+ required to form the characteristic octahedral coordination in rutile. The mixed oxides exhibit enhanced stability in both acidic and alkaline environments under anodic potentials with no changes in their crystal structure after extensive electrochemical stress. The improved stability of these mixed oxides highlights their potential application as scaffolds to host and stabilize OER active metals.

Tensile straining of iridium sites in manganese oxides for proton-exchange membrane water electrolysers

Although the acidic oxygen evolution reaction (OER) plays a crucial role in proton-exchange membrane water electrolysis (PEMWE) devices, challenges remain owing to the lack of efficient and acid-stable electrocatalysts. Herein, we present a low-iridium electrocatalyst in which tensile-strained iridium atoms are localized at manganese-oxide surface cation sites (TS-Ir/MnO2) for high and sustainable OER activity. In situ synchrotron characterizations reveal that the TS-Ir/MnO2 can trigger a continuous localized lattice oxygen-mediated (L-LOM) mechanism. In particular, the L-LOM process could substantially boost the adsorption and transformation of H2O molecules over the oxygen vacancies around the tensile-strained Ir sites and prevent further loss of lattice oxygen atoms in the inner MnO2 bulk to optimize the structural integrity of the catalyst. Importantly, the resultant PEMWE device fabricated using TS-Ir/MnO2 delivers a current density of 500 mA cm-2 and operates stably for 200 h.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

MnO2-Ir Nanowires: Combining Ultrasmall Nanoparticle Sizes, O-Vacancies, and Low Noble-Metal Loading with Improved Activities towards the Oxygen Reduction Reaction

Although clean energy generation utilizing the Oxygen Reduction Reaction (ORR) can be considered a promising strategy, this approach remains challenging by the dependence on high loadings of noble metals, mainly Platinum (Pt). Therefore, efforts have been directed to develop new and efficient electrocatalysts that could decrease the Pt content (e.g., by nanotechnology tools or alloying) or replace them completely in these systems. The present investigation shows that high catalytic activity can be reached towards the ORR by employing 1.8 ± 0.7 nm Ir nanoparticles (NPs) deposited onto MnO₂ nanowires surface under low Ir loadings (1.2 wt.%). Interestingly, we observed that the MnO₂-Ir nanohybrid presented high catalytic activity for the ORR close to commercial Pt/C (20.0 wt.% of Pt), indicating that it could obtain efficient performance using a simple synthetic procedure. The MnO₂-Ir electrocatalyst also showed improved stability relative to commercial Pt/C, in which only a slight activity loss was observed after 50 reaction cycles. Considering our findings, the superior performance delivered by the MnO₂-Ir nanohybrid may be related to (i) the significant concentration of reduced Mn³⁺ species, leading to increased concentration of oxygen vacancies at its surface; (ii) the presence of strong metal-support interactions (SMSI), in which the electronic effect between MnO_x and Ir may enhance the ORR process; and (iii) the unique structure comprised by Ir ultrasmall sizes at the nanowire surface that enable the exposure of high energy surface/facets, high surface-to-volume ratios, and their uniform dispersion.